tweak: iteration over element and index in scratchpad.

src/scratchpad/for_seqs.nim

@@ -0,0 +1,8 @@
+proc p(xs: seq[string]): int = 
+  for i, x in xs:
+    if x == "c":
+      return i
+  return -1
+
+let xs = @["a", "b", "c"]
+echo p(xs)

src/scratchpad/seqs.nim

@@ -1 +1,2 @@
 echo @[ "1", "2" ] in @["1", "2", "3" ]
+# ^ does not work.